Generally, mobile communication systems have been developed to provide communication services while guaranteeing mobility of users. With the dramatic development of technologies, the mobile communication systems are now capable of providing high-speed data communication services as well as voice communication services. Recently, standardization work on Long Term Evolution (LTE), one of the next-generation mobile communication systems, is in progress in 3rd Generation Partnership Project (3GPP).
LTE is a technology for implementing packet-based communication at a higher data rate of a maximum of about 100 Mbps than a currently provided data rate, aiming at commercialization in around 2010. For data services, unlike in voice services, allocatable resources are determined according to an amount of transmission data and a channel status. Thus, in wireless communication systems such as mobile communication systems, a scheduler is in charge of management by allocating transmission resources considering an amount of the transmission resources, a channel status, and an amount of data. Such a scheme is also applied to the LTE which is one of the next-generation mobile communication systems, and a scheduler located in a base station manages and allocates wireless transmission resources.
In recent, active discussion has been going on about LTE-Advanced (LTE-A) communication systems which enhance a transmission speed by introducing various new technologies in the LTE communication systems. Carrier Aggregation (CA) is a representative one of the introduced new technologies. According to CA, unlike a conventional scheme where a User Equipment (UE) performs data transmission/reception by using a single downlink carrier and a single uplink carrier, a UE uses multiple downlink carriers and multiple uplink carriers. In this way, by allocating multiple carriers to a UE, the transmission speed and data rate of the UE can be increased.
FIG. 1 is a diagram illustrating the architecture of a general LTE mobile communication system.
Referring to FIG. 1, a radio access network of the LTE mobile communication system includes Evolved Node Bs (ENBs), also called Node Bs, 105, 110, 115, and 120, a Mobility Management Entity (MME) 125, and a Serving-Gateway (S-GW) 130.
A UE 135 accesses an external network through the ENBs 105, 110, 115, and 120, and the S-GW 130. The ENBs 105, 110, 115, and 120 correspond to entities in a form where conventional Node Bs of a Universal Mobile Telecommunications System (UMTS) and a Radio Network Controller (RNC) are combined with each other. The ENBs 105, 110, 115, and 120 are connected with the UE 135 over a wireless channel, and play more complex roles than the conventional Node Bs. In LTE, all user traffics including real-time services such as Voice over IP (VoIP) will be serviced over a shared channel. This means that there is a need for an apparatus of collecting status information of UEs and performing scheduling depending thereon, and the scheduling is managed by the ENBs 105, 110, 115, and 120. The ENBs 105, 110, 115, and 120 also control radio resources of cells. A single ENB typically controls multiple cells. To realize a data rate of a maximum of about 100 Mbps, LTE uses Orthogonal Frequency Division Multiplexing (OFDM) as radio access technology in a 20-MHz bandwidth. In addition, an Adaptive Modulation & Coding (AMC) scheme of determining a modulation scheme and a channel coding rate according to a channel status of UEs is applied to LTE. The S-GW 130 is a device for providing data bearers and creates or removes a data bearer under control of the MME 125. The MME 125 is a device in charge of various control functions and is connected with the multiple ENBs 105, 110, 115, and 120.
FIG. 2 is a diagram illustrating an embodiment of a UE for which multiple carriers are aggregated.
It is general that in a single base station, multiple carriers located in different frequency bands are transmitted and received. For example, when a DL carrier_1 201 having a center frequency F1 and a DL carrier_2 221 having a center frequency F4 are transmitted, a single UE conventionally receives data from one of the two carriers 201 and 221; whereas a UE having a Carrier Aggregation (CA) capability can receive data from several carriers at the same time. That is, the UE shown in FIG. 2 can receive data from both the DL carrier_1 201 and the DL carrier_2 221 at the same time. Also in case of uplink (UL) transmission, a UE conventionally transmits data through a single carrier; whereas a UE having the CA capability can transmit UL data through both a UL carrier_1 211 and a UL carrier_2 231 at the same time. The base station allocates more carriers to a UE having a carrier aggregation capability, depending on circumstances, thereby improving the transmission speed of the UE. Assuming that a single DL carrier and a single UL carrier form a single cell in a traditional sense, CA may be understood as simultaneous data transmission/reception of the UE through several cells. In this way, a conventional maximum transmission speed in a single cell can be increased proportionally to the number of carriers aggregated for the UE having the CA capability.
FIG. 3 is a diagram showing the necessity and roles of UL timing synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system.
OFDM is a multiplexing technique which divides a broadband frequency channel into multiple narrowband channels for transmission. OFDM is often used in a 3GPP LTE mobile communication system as a modulation technique.
Referring to FIG. 3, UE1 denotes a UE which is located in adjacent to an ENB and UE 2 denotes a UE which is located far from the ENB. T_pro1 denotes a propagation delay time in radio transmission to the UE1, and T_pro2 denotes a propagation delay time in radio transmission to the UE2. The UE1, because of being located nearer the ENB than the UE2, has a shorter propagation delay time. In FIG. 3, T_pro1 is equal to 0.33 μs and T_pro2 is equal to 3.33 μs.
In a cell of the ENB as shown in FIG. 3, when the UE1 and the UE2 are powered on or they are in the idle mode, a UL timing of the UE1, a UL timing of the UE2, and UL timings of UEs in a cell detected by the ENB do not match one another. Reference numeral 301 denotes a symbol for UL OFDM symbol transmission of the UE1, and reference numeral 302 denotes a symbol for UL OFDM symbol transmission of the UE2. Taking account of the propagation delay times of the UL transmissions of the UE1 and the UE2, timings for UL OFDM symbol reception of the ENB from the UE1 and the UE2 are 312 and 313. That is, the UL symbol 301 of the UE1 is received by the ENB at the timing 312 with a propagation delay time (T_pro1) of 0.333 μs, and the UL symbol 302 of the UE2 is received by the ENB at the timing 313 with a propagation delay time (T_pro2) of 3.33 μs. As shown in FIG. 3, since the timings 312 and 313 precede synchronizing the UL timing of the UE1 and the UL timing of the UE2, a start timing 311 for receiving and decoding an UL OFDM symbol by the ENB, the timing 312 for OFDM symbol reception from the UE1, and the timing 313 for the OFDM symbol reception from the UE2 are different from one another. The UL symbols transmitted from the UE1 and the U2 do not have orthogonality with respect to each other, thus acting as interference with each other, and the ENB cannot successfully decode the UL symbols 301 and 302 transmitted from the UE 1 and the UE2 due to the interference and the UL symbol reception timings 312 and 313 which do not match the start timing 311.
Therefore, an UL timing synchronization procedure is performed to synchronize the UL symbol reception timings of the UE1, the UE2, and the ENB, and upon completion of the UL timing synchronization procedure, the start timing 321 for receiving and decoding the UL OFDM symbol by the ENB, the timing 322 for UL OFDM symbol reception from the UE1, and the timing 323 for UL OFDM symbol reception from the UE2 match one another. By synchronizing the UL timings in this way, the UL symbols transmitted from the UE1 and the UE2 can maintain orthogonality, and thus the ENB can successfully decode the UL symbols transmitted from the UE1 and the UE2 according to the timing 301 and the timing 302.
The UL timing synchronization procedure for acquiring UL timing synchronization uses a random access procedure which is carried out in a target cell when a UE transits from a Radio Resource Control (RRC) idle mode to an RRC connected mode or after handover is performed.
FIG. 4 is a diagram illustrating an embodiment of the random access procedure for acquiring UL timing synchronization, carried out in a target cell after handover.
Referring to FIG. 4, reference numeral 401 denotes a UE, reference numeral 403 denotes a source cell where the UE 401 is located prior to handover, and reference numeral 405 denotes a handover target cell. The UE 401 first performs measurement report to the source cell in step 411. The measurement report involves measuring a channel status of a current cell and a channel status of a neighboring cell according to a configuration set by the ENB of the source cell 403, that is, a particular event or period-related information triggering the management report, and providing the ENB with the measurement result. The ENB makes a decision on the handover of the UE 401 in step 421. If the ENB of the source cell 403 decides to hand over the UE 401 to a target cell 405 in step 421, the ENB of the source cell 403 sends a handover command message to the UE 401 to instruct the UE 401 to hand over to the target cell 405 in step 431. The handover command message includes a cell ID information of the target cell 405 and radio channel reconfiguration information reconfigured for use by the UE 401 in the target cell 405. The radio channel reconfiguration information includes radio resource information to be used by the UE 401 in the target cell 405 and Cell Radio Network Temporary ID (C-RNTI) to be used in the target cell 405.
The UE 401, upon receiving the handover command message from the source cell 403 in step 431, sends a random access preamble, which is a code sequence, through a Random Access Channel (RACH) to the target cell 405 to synchronize a UL timing with the target cell 405 in step 441. The ENB for controlling the target cell 405 may know an UL Timing Advance (TA) between the UE 401 and the ENB through the random access preamble reception. The ENB provides the TA to the UE 401 in step 443, and the UE 401, upon receiving the TA, corrects an UL timing based on the TA and sends a handover confirm message indicating completion of the handover to the target cell 405 by using the corrected UL timing in step 451. Allocation of a UL radio resource for sending the handover confirm message may be notified when the UE 401 is informed of the UL TA in the target cell 405. After sending the handover confirm message, the UE 401 transmits and receives data to and from the target cell 405 in step 461.
FIG. 5 is a diagram illustrating an embodiment of a scenario of a UE for which multiple UL carriers requiring different UL TAs are aggregated.
Referring to FIG. 5, reference numeral 501 denotes an ENB, and reference numeral 511 denotes a UE for which multiple UL carriers F1 and F2 are aggregated. The UE 511, when performing UL transmission by using the UL carrier F1, transmits data directly to the ENB 501 as indicated by 531. When performing UL transmission by using the UL carrier F2, the UE 511 transmits data to the ENB 501 through a repeater 521 as indicated by 541. As a result, the UL transmission delay times to the ENB through the UL carrier F1 and the UL carrier F2 are different from each other, and thus UL TAs for the UL carrier F1 and the UL carrier F2 are also different from each other.